Is it pay to win?
I'd played this game back when Warframe was still picking up. The devs have done an amazing job improving upon this game. It even won Steam's Labor of Love award last year.
The term "Pay to Win" isn't as subjective as some want to believe. It's often overly used *incorrectly* by people who are just too lazy to put any work into a game, and assume that paying would be faster progress. Every game out there has some form of premium shop or subscription, because, guess what? The devs actually have to make a profit somehow. Crazy, right? The game industry actually runs on money. Wow. Mind blown.
This game isn't pay to win. The premium subscription simply adds a feature that many beg for in other games: the option to spend less time grinding when you have a busy schedule. Grown adults play video games too, but may be too busy to spend months grinding for gear. But not having it doesn't kill the fun. Some people do have the time, and enjoy grinding for gear. This game is whatever people want the game to be for them, and that's the whole point of gaming.
If you want the game to be fully sustainable, you trade mats for plats. It used to be that you had to hit up trade chat and sell old-school, but I'm not sure if they changed that, since I haven't looked into that yet.